Some cartridges can apparently be damaged by active demagnetizing. VdH specifically advises against it for example. I don't know that it would hurt a ZYX, but to actively demag any cartridge without checking that it's safe would be imprudent. I'd do what Rgurney said first.
The Cardas record's demag tracks are simple frequency sweeps. Why playing a two minute frequency sweep demags anything is beyond me, but it's even harder imagining it doing any harm.
